1. Field of the Invention
This invention relates to a planar liquid filled optical waveguide, which has particular but not exclusive application as a fast all optical switching device for time division multiplexing (TDM) network architectures and sub-systems.
2. Related Art
In a TDM network, low bit rate data streams, typically at 64 kbits/sec per telephony channel, derived from digital sources such as modems and digitally sampled analogue sources such as telephones, together with other information such as billing data, are multiplexed for transmission across the network by interleaving the signals in time to create a much higher bit rate signal, at up to 1 Gbit/sec or greater, but typically at 500 Mbits/sec. All-optical switching devices may be used in such systems to provide functions such as optically controlled modulation and all-optical clock signal recovery in high frequency systems.
Liquid filled optical waveguides are known. Reference is directed to our Publication Number WO 96/10282; "Optical fibre with quantum dots", in which a hollow tubular glass cladding is filled with a colloidal solution of quantum dots through an opening at either end. The principle extends to a planar waveguide where, for example, the waveguide is a channel formed in a substrate.
Although the method of end filling is suitable for short lengths of waveguide, it becomes increasingly difficult to fill longer lengths in this way. This is partly due to the viscous drag at the liquid/waveguide interface and partly the increasing chance that impurities at this interface will cause only partial wetting, which may stop the filling process entirely. Longer waveguide lengths are desirable as they may enable the device to operate with lower powered sources than would otherwise be possible, in turn reducing the risk of optical damage to certain materials which have useful optical properties and which may be used in the construction of such devices. Whilst the application of reduced or increased pressure may be used to assist the process of end filling, this may not be possible with materials such as volatile liquids.
U.S. Pat. No. 5,473,721 discloses a multi-mode optical circuit in which channels for receiving optical fibres are machined into a substrate. The channels can be filled with liquid through transversely arranged filling passages, the liquid in the wave guiding region so formed being confined by the ends of the optical fibres inserted into the machined channels. U.S. Pat. No. 5,394,239 discloses a solid optical waveguide in the form of a coil on a substrate.